The present review aims to illustrate current knowledge about the efficacy of omega-3 long-chain polyunsaturated fatty acids (n-3 LC-PUFAs) in treating/preventing several metabolic pathologies. We reviewed systematically the published evidence on the effectiveness of n-3 LC-PUFAs fish consumption or n-3 LC-PUFAs supplementation on prevention/treatment of obesity, metabolic syndrome, and cardiovascular diseases. Most of the reviewed studies were randomized-controlled interventional trials, although some relevant prospective and cross-sectional studies as well as some meta-analysis were also reviewed. Supplementation with n-3 LC-PUFAs might improve some obesity-associated metabolic syndrome features such as insulin resistance, hypertension and dyslipidemia by decreasing plasma triglycerides. Moreover, the blood pressure-lowering and anti-inflammatory properties of these fatty acids and their benefits in vascular function might confer cardioprotection. However, the efficacy of n-3 LC-PUFA on reducing myocardial infarction, arrhythmia, cardiac and sudden death, or stroke is controversial. Due to the beneficial actions of n-3 LC-PUFAs, several worldwide government and health organizations have established some recommendations of n-3 LC-PUFAs intake for groups of population. In general, the recommended levels for diseases prevention are lower than those advised for particular treatments. However, more clinical trials are necessary to recommend the most effective dosages and formulas (type of n-3 LC-PUFA, EPA/DHA ratio) for specific pathologies.